I installed Windows 10 as normal on a 240GB SSD, leaving aside a few gigs for swap.

Then I installed LMDE 2 onto a separate 32 GB M.2 drive. I noted that that /boot/efi was pointed at the Windows system partition by default, and swap was pointed at the partition I had set aside, but otherwise this installation is on a separate physical disk.

I have an ASUS-Z170-A motherboard with Secure Boot disabled, but otherwise default settings.

No matter what I do, the only way to boot into the different operating systems is to select or unselect the Windows Boot Manager as my first boot device in the motherboard firmware. Even if I select the 240GB SSD, it gives me Grub, but no Windows. It has to be the Windows Boot Manager or I'll end up in LMDE.

In my experience GRUB does not detect windows when it was installed on the wrong HDD. it has to be on the same drive as the MBR (Master Boot Record) which is usually the Windows HDD. Done install it on a partition, just the Master Windows drive.